Silver plating baths

ABSTRACT

Novel stable aqueous cyanide-plating baths are provided which include silver ions, sodium ions and cyanide ions, in which the OH normality, the &#39;&#39;&#39;&#39;free&#39;&#39;&#39;&#39; CN normality, the sodium cyanide normality and the total silver normality are interrelated according to a specified formula. Novel procedures for electroplating are also provided.

United States Patent Dingley et al.

[4 Feb. 29, 1972 [541 SILVER PLATING BATHS [72] Inventors: Wilfred Dingley, Ottawa; John S. Bednar,

Gatineau Point; Raymond R. Rogers, Ottawa, all of Canada [73] Assignee: Canadian Patents and Development Limited, Ottawa, Ontario, Canada [22] Filed: July 3, 1969 [21 Appl. No.: 839,003

[52] (1.8. CI ..204/34, 204/46 [5 1] Int. Cl. C23b 5/62 [58] Field of Search ..204/34, 38 B, 46; 117/130 E [56] References Cited UNITED STATES PATENTS Attorney-Milieu, Raptes & White l-lelmke 2O4/46 Primary Examiner-John H. Mack Assistant Examiner-William 1. Solomon [57] ABSTRACT jNovel stable aqueous cyanide-plating baths are provided electroplating are also provided.

27 Claims, 3 Drawing Figures PAIENTE0FEB29 I972 SHEET 1 BF 2 WILFRED DINGLEY JOHN s BEDNAR RAYMON D R ROGERS INVENTORS ATTORNEY SHLVER PLATING BATHS This invention relates to electroplating silver onto a metal substrate. More particularly, it relates to novel stable silver cyanide plating baths, and to novel procedures for electroplating using such baths.

BACKGROUND OF THE lNVENTlON Silver has important electrical, mechanical and chemical properties and it would be useful as a coating on many types of metal products. Until recently, however, silver plating has been done mostly for decorative purposes in which the highest quality of product has not been essential. Nowadays, however, industry is demanding higher quality of silver plating because the plated parts must be serviceable under more severe conditions for a longer period of time, and sometimes must have high electrical conductivity and low electrical contact resistance as well.

The most important chemicals required in producing present-day plating baths are silver cyanide (AgCN) and potassium cyanide (KCN), although potassium hydroxide and potassium carbonate are sometimes added. Copper cyanide (CuCN) is an important constituent of the first strike bath used when plating silver on steel. Potassium compounds are commonly used instead of the cheaper sodium compounds, because they have a higher electrical conductivity and are generally purer. The silver is believed to be present in the plating bath in the form of the complex ion Ag(CN) Any cyanide in the bath, in addition to the compound KAg(CN is termed free cyanide. One serious disadvantage of these present-day baths is that such baths change their compositions fairly rapidly (i.e., are unstable) during plating, the cyanide content decreasing and the carbonate content increasing.

Although silver may be electroplated on many metals and alloys, the present invention is directed to the embodiments where steel and copper are used as the basis metals. Steel, because of the large quantity of this metal used in industry and also because this operation has been particularly difficult to do satisfactorily; copper, since silver-plated copper is extensively used in industry. When steel, unconnected to a source of electricity, is immersed in ordinary present-day silver plating baths, it becomes covered with a very poorly adhering deposit of silver. To ensure that this does not occur during the actual plating operation, the following complicated threestage process has generally been used heretofore:

l. Plate the steel in a copper-silver strike bath having a comparatively high free" cyanide content. This contains more copper than silver, and the resulting thin metal film consists of an alloy containing a greater proportion of copper than of silver. Such a film adheres fairly well to the basis metal.

2. Plate the product from (1) with a thin film of silver in a silver strike bath that contains no copper but does contain a comparatively high free" cyanide content.

Plate the product from (2) in a more concentrated bath operating at a higher cathode current density and current efficiency.

Unfortunately, the above-described operation produces embrittlement in certain steels, such as Types 1062 and 4037, which are of the high strength variety. Pins of these steels, plated in this way, break on bending. The commonly accepted explanation for the effect of hydrogen embrittlement is the reduction of ductility so that the metal tends to fail by brittle fracture rather than by plastic deformation.

It appears that there are no ferritic steels which are immune to hydrogen embrittlement, but there are rather wide variations in the degree of susceptibility of the many types of steel that fall into this category. In general, these variations can be correlated with the composition and microstructure of the steel. The embrittlement occurs at all strength levels and it is usually most severe at what is termed ultrahigh strength levels, commonly namely about 200,000 p.s.i. and above.

In other words, the problem of hydrogen embrittlement of steel parts which are finished by electroplating becomes acute when medium and high-strength steels must be processed. Generally, steels having an ultimate tensile strength of 150,000 p.s.i. or less are not significantly affected by the hydrogen to which they are exposed during finishing. Steels with strengths in the neighborhood of 180,000 to 220,000 p.s.i. are, however, adversely affected and are subject to premature failure. Those steels in the range of 260,000 to 280,000 p.s.i. are usually so susceptible to hydrogen as to be unsafe for structural use unless special precautions are taken to relieve or prevent their embrittlement.

PRIOR PROPOSALS There have been many suggestions for solving the embrittlement problem in steel. One well-known way to remove hydrogen embrittlement is baking. Baking is a diffusion process and as such is controlled by time and temperature. Because the rate of diffusion increases exponentially with an increase in temperature, it is always desirable to bake at the highest possible temperature thereby reducing markedly the time required. Production costs will also be lowered because oven time will be decreased. Baking is thus an added expense and moreover, it is not always effective. Other metallurgical considerations must be taken into account, however, in choosing a baking temperature. Almost all alloys which are likely to be embrittled are either heat-treated or cold-worked to produce certain properties. Obviously, a baking temperature high enough to remove even partially the effects of such treatment must be avoided.

Avoidance of hydrogen embrittlement in the first place is, of course, the most desirable state of affairs. Two avenues of approach are open, namely: firstly, employing those processes which do not produce hydrogen embrittlement; and secondly, applying a barrier coating to the basis metal prior to an operation which normally produces embrittlement.

Some of the techniques which have been suggested for solving the embrittlement problem include the use of thin coatings deposited at high current density. Such coatings are porous and any embrittlement encountered may usually be relieved by baking after plating. This method, however, makes the process more complicated and has the disadvantages that baking may not entirely relieve embrittlement and hydrogen can also reenter through pores.

A further technique suggested has been the use of vacuumevaporated coating which are not embrittling. At present, these are very expensive and cannot be used for large parts in currently available apparatus.

A further procedure which has been suggested is a multiple step plating but it has been felt that this is notdependable from a processing standpoint since it does not uniformly prevent embrittlement.

It has also been suggested that the high-strength steels should be cleaned in an exacting manner prior to .electrodeposition. It has been found that conventional cleaning processes which involve the use of strong acids or alkalis cannot be used with the susceptible steels unless the hydrogen which they introduce is subsequently baked out. Because baked surfaces are oxidized and unfit for plating, it was found to be necessary to use other preplate cleaning methods. For medium and high-strength steel articles, it has been suggested that baking may be used to relieve any embrittlement before preplate cleaning is started. Furthermore, as the surfaces to be plated must be free of contamination and must not be processed in any manner likely to cause hydrogen embrittlement, the treatment of the steel becomes critical. The medium strength steels were, therefore, proposed to be handled as little as possible. It was suggested that they be dry sandblasted, airblown or rinsed and then plated immediately. Particularly careful handling of the parts after sandblasting to avoid fingerprints or any other surface contamination, was suggested as being essential.

For high-strength steel parts, anodic alkaline cleaning-followed immediately by a water rinse, was recommended after the preliminary baking. However, it was found that enough embrittlement normally occurred in an alkaline cleaning bath without the anodic current to preclude its use on high-strength steels. The parts, therefore, must be kept anodic in the bath and must be rinsed immediately after being cleaned. It was also suggested that if there was any visible surface contamination after this step, the parts be air-dried, and then be cleaned further by blasting with clean dry abrasive and again waterrinsed. It was also suggested that they next be electrohoned by the use of a sulphuric-phosphoric acid bath and immediately water-rinsed. It was then suggested that the parts be immersed in a plating solution for minutes without current and then be finally electroplated in the same solution. Because of this anodic alkaline cleaning treatment, however, it is desirable to provide a more economical process.

The importance of having the high-strength steel parts absolutely clean before plating cannot be overemphasized. It is observed that even a thin film left on the steel by handling the parts without gloves may cause marked embrittlement. Also the parts must be thoroughly rinsed immediately after the anodic alkaline cleaning and the electrohoning as it was found that residual alkali or acid on the surface can produce embrittlement.

AIMS OF THE INVENTION Accordingly, an object of one aspect of this invention is the provision of stable silver plating baths in which there is a comparatively simple control of the bath composition.

An object of yet another aspect of this invention is the provision of stable silver plating baths which, when used for electroplating, provide a superior appearance of the electroplated silver even in the absence of brighteners.

An object of a still further aspect of the present invention is the provision of stable silver plating baths which, when used for electroplating, have the ability of providing insolubility of the silver anodes in the plating baths when the latter are not in use.

An object of yet another aspect of this invention is the provision of stable silver plating baths which, when used for electroplating, have the ability of substantially completely obviating silver deposition by replacement on unconnected steel cathodes, when the latter are immersed in the plating baths.

An object of another aspect of this invention is to provide a process for electroplating silver in which there is an improvement in the operating characteristics of the plating cell.

An object of yet another aspect of this invention is to provide an improved process for electroplating silver in which a higher than heretofore used current density may be practically used.

An object of still another aspect of this invention is to provide an improved process for electroplating silver in which a greater thickness of the electroplated silver per ampere-hour may practically be achieved.

An object of still another aspect of this invention is the provision of an improved process for electroplating silver which obviates preliminary plating in strike baths when plating on steel.

An object of yet another aspect of this invention is the provision of an improved process for electroplating silver wherein there is no significant embrittlement of high-strength steels.

BROAD STATEMENTS OF THE INVENTION By one broad aspect of this invention, stable silver cyanide plating baths are provided comprising ions of silver, ions of sodium and ions of cyanide, the normalities of the constituents of each such bath being related to one another by the following equations:

Y=OH normality, and

X=free" CN' normalityyand Total sodium cyanide normality Between 6.1 and 8m 7 7 g Total sllver normality l wherein Y=0H normality, and

X=free CN' normality; and

( Total sodium cyanide normality Between 6-1 and SJ Total silver normality By yet another aspect of this invention, a process is provided for plating high-strength steel with silver, the process comprising the steps of: I. pretreating said steel by producing or maintaining a clean smooth substantially oxide free surface on the steel which is conducive to being electroplated with silver without causing hydrogen embrittlement, by one of the steps of (A) providing a cleaned surface, smooth surface, substantially free of oxide film, then treating the surface with an acid cleaning solution consisting essentially of nitric acidacetic acid-phosphoric acid, thereby to maintain said smooth, clean surface, and finally removing smut from the acid washed steel surface, and (B) cleaning away oxide film from the steel surface, then producing, electrolessly, a thin fugitive porous coating of copper on the steel surface, then removing that metal coating and simultaneously producing a smooth surface by washing with an acid-cleaning solution containing nitric acid and acetic acid, and finally removing smut from the steel surface; and II. electroplating the silver coating on the pretreated steel from a stable silver cyanide bath comprising: ions of silver, ions of sodium and ions of a cyanide, the normalities of the constituents of the bath being related to one another by the following equations:

Y=0.374+0.768X0. l78X Total sodium cyanide normality 6.1 and SJ Total silver normality V By an embodiment of this aspect of the present invention, step I includes the steps of: (a) cleaning said steel surface with hydrochloric acid; (b) depositing, electrolessly, on said steel a thin porous copper flash; (c) contacting said deposited copper surface with an acid cleaning solution containing nitric acid and acetic acid; and (d) cleaning said surface resulting from step (c) with water.

By still another aspect of this invention, a process is provided for producing a stable silver cyanide plating bath, the stable plating bath to be produced having a composition satisfying the following conditions, namely the normalities of the constituents of the bath being related to one another by the following equations:

Y=0.374+0.768X().l78X

(l wherein determined total sodium cyanide normality, the free" CN' normality is determined from the curve in FIG. 3; and from that determined free CN normality, the OH" normality is determined from the curve in FIG. 1; placing silver anodes and metal cathodes in the bath; and slowly agitating and electrolyzing the bath for a period of time until a stable bath is formed in which the free" CN normality is substantially constant.

By another aspect of this invention, a process is provided for restabilizing a formerly stable silver cyanide plating bath which comprises: selecting a bath whose composition formerly satisfied the following conditions, namely, that the normalities of the constituents of the bath are related to one another by the following equations:

Y=0.El74+0.768X0.178X I wherein Y=OH normality, and

X=free CN normality; and

( Total sodium cyanide normal1ty 6-1 and 8.1

Total silver normality and placing said bath in a container; readjusting the normalities of the constituents so that, for a predetermined and selected silver cyanide normality, the total sodium cyanide normality is determined from the curve in FIG. 2; for that determined total sodium cyanide normality, the free (,N normality is determined from the curve in FIG. 3; and from that determined free CN" normality, the 0H normality is determined from the curve in FIG. I; placing a silver anode, and a metal cathode into the bath; and slowly agitating and electrolyzing the bath for a period of time until a stable bath is formed in which the free CN normality is substantially constant.

THE DRAWINGS In the accompanying drawings,

FIG. 1 is a graph of 0H normality as abscissa, and free CN' normality as ordinate, showing the relationship between the free CN and 0H normalities in stable silver-plating baths of aspects of this invention;

FIG. 2 is a graph of AgCN normality as abscissa, and total NaCN normality as ordinate, showing the relationship between the normality of the AgCN and that of the total NaCN in the initial baths from which the stable baths of aspects of this invention are produced; and

FIG. 3 is a graph of total NaCN normality as abscissa and free CN normality as ordinate, showing the relationship between the normality of the total NaCN required in an initial bath and that of the free" CN required in a final stable bath of an aspect of this invention.

GENERAL EXPERIMENTAL PROCEDURES Before providing examples of aspects of the present invention, applicant desires to include a brief description of the materials, equipment and general procedures used.

ELECTRODES The cathode materials used in the examples provided hereunder were as follows:

1. Sheets cut from zinc-coated, low-carbon-steel Hull cell panels, to a size of 6.35 cm. (2.5 in.)Xl.27 cm. (0.5 in.)X0.025 cm. (0.01 in.). (The zinc coating was removed before the silver plating was begun.)

2. Type 1062 high-carbon-steel pins, 8.26 cm. (3.25 in.) long and 0.38 cm. (0.15 in.) in diameter. They had been produced from drawn wire and austempered in the bainitic range, resulting in a Rockwell C hardness of 52-56 and an ultimate strength of 257-279 k.p.s.i. They were covered with a thin uniform scale of blue iron oxide when received.

TABLE 1 Steel Analyses (Percent) Constituent Type of Steel (AISI) Carbon 0.64 0.40 Manganese 1.03 0.76 Silicon 0.16 0.3l

Molybdenum 0.26 Sulfur (max) 0.009 0.02 Phosphorus 0.002 0.02

The anodes used in the examples provided hereunder were of bar silver specially produced for use in electroplating. The immersed portion of each one was 12.7 cm. (5 in.)X2.5 cm. (1.0 in.) 0.6 cm. (0.2 in.). They had been purchased from Johnson Matthey and Mallory Ltd.

CATHODE SURFACE PREPARATION PRIOR TO SILVER PLATING Before silver plating, each sheet cathode was treated as follows:

1. Stripped of zinc by immersion in 18 percent (by weight) hydrochloric acid produced by diluting concentrated chemically pure acid with distilled water. This operation was performed at room temperature and was continued until vigorous gas evolution had ceased.

2. Rinsed thoroughly in tap water.

3. Introduced into the plating bath with as little delay as possible.

The steel to be electroplated must be pretreated in order to provide a surface which can accept the silver plating without causing hydrogen embrittlement. Generally speaking, if the steel has a smooth, clean, oxide free surface, the pretreatment used is to maintain such surface while making the surface conducive to accepting the silver plating without causing hydrogen embrittlement. In such circumstances, the surface of the steel may have been suitably formed in a nonoxidizing atmosphere to be smooth, clean and oxide free. Alternatively, it may have been acid washed, e.g., with hydrochloric acid or with sulphuric acid. Then, the steel surface would be washed with an aqueous solution containing 23.3 percent by weight of nitric acid, from 31.1 to 34.3, preferably 32.7 percent by weight of acetic acid, and from 27.9 to 29.7, preferably 28.3 percent by weight of phosphoric acid, the balance being water which was contained in the nitric, acetic and phosphoric acids. Finally, smut on the steel surface is removed. This can be done by rinsing with water under sufficient hydraulic or pneumatic pressure, or by mild rinsing in conjunction with ultrasonic vibration, or by air-water blasting.

On the other hand, if the surface of the steel is not smooth, clean and oxide free, it may l0treated firstly to remove the visible, i.e., oxide film, as by shot peening, sandblasting or the above-described acid washing. Then a very thin porous coating of copper is electrolessly plated on. In the case of copper, this may be achieved by treatment in a copper sulfate solution by a straight replacement of iron by copper following conventional techniques. Then the fugitive film is removed while simultaneously forming a smooth steel surface conducive to being electroplated with silver without causing hydrogen embrittlement by treatment with an aqueous nitric acid-acetic acid solution, e.g., one having a ratio of from 10 15 percent by weight of nitric acid and from 4 to about 35 percent by weight acetic acid, with the balance being water. Then the smut is removed as described above.

Each pin of Type 1062 steel thus was treated by the following procedure which, it has been found, produced a surface that is thoroughly clean, smooth, active, and substantially free from embrittlement.

2. Degrease in degreasing substance, e.g., the vapor of suitably stabilized trichlorethylene.

2. Clean away visible films of foreign materials, i.e., oxide film, from the steel surface, e.g., by immersion in hydrochloric acid (e.g., 18 percent by weight), while under the influence of ultrasonic vibration for an optimum time of 1-3 minutes. (This was done in an LT-6O transducerized tank energized with a Sonogen ultrasonic generator Model LG-l50 operating at 25 kc. and 150 watts. Both equipments were obtained from Branson Ultrasonic Corp.

3. Rinse clean, e.g., by rinsing in flowing tap water.

4. Treatment in copper sulfate solution to produce a very thin copper coating on the steel, e.g., by immersion in a copper sulfate (CuSO 511 (e.g., 10 percent by weight) solution of pH 2.2-2.8, e.g., 2.6.

. Rinse clean, e.g., by rinsing in flowing tap water.

. Treatment with an acid cleaning solution including nitric acid and acetic acid to produce a smooth surface free from minute sharp edges without materially increasing the temperature of the acid solution and simultaneously to remove the copper coating. Examples of suitable solutions are as follows:

(A) nitric acid: 10-15% by weight 28-31% by weight balance.

phosphoric acid:

water:

For example, therefore, the steel is then immersed in a solution containing nitric acid (7 percent by weight) and acetic acid (14 percent by weight) for 3 to 5 minutes. During this treatment, the copper coating and other impurities on the steel surface were loosened or removed, and the roughness was eliminated.

7. Rinse clean, e.g., by rinsing in water while under the influence of ultrasonic vibration. This treatment removes smut and copper remaining on the surface after the previous treatment.

8. Introduce into the plating bath with as little delay as possible.

Each pin of Type 4037 steel was treated by a procedure consisting of steps I, 6 and 7 of the treatment used with the Type 1062 pins. Steps 2 to 5 were not used, because these pins were copper-coated when received.

An alternative surface treatment of high-strength steel which may be used is described in the Journal of the Electrochemical Society Vol. 1 16 No. l, 1968, pages 130-133 by A. W. Lui and R. R. Rogers. Briefly, it involves making the steel anodic in 0.05-0.2N I-ICl for a time less than 10 minutes to minimize hydrogen embrittlement. The potential may be O.5 to 0.4 volts with respect to a saturated calomel electrode.

ELECTROPLATING This operation in each of the examples hereunder was performed in a plating bath of 800 ml. volume in a cylindrical glass container. Both vertical edges of the single flat silver anode were placed against one side of the container. Each sheet steel cathode, suspended by means of a silver-plated steel clip, was placed at an angle of 90 to, and with its near edge 3.8 cm.(l.5 in.) from, the anode. Under these circumstances, the silver was plated on both sides of the cathode and it was dissolved from only one side of the anode. The ratio Effective anode area When Type 1062 or Type 4037 pins were used as cathode, this ratio was 4.7 or 6.3 respectively.

The plating bath was agitated, slowly by a steel propeller coated with an inert polytetrafluoroethylene polymer known by the Trade Mark of Teflon. The DC electricity for plating was supplied by a selenium rectifier. One series of cathodes was plated at a temperature of 25:2 C. (77 F.) and a second one at 49fl C. F.).

Each plating bath was prepared by dissolving reagent-grade chemicals in distilled water and adding activated carbon. The mixture then was electrolyzed, using a metal cathode, e.g., a mild steel cathode, and a silver anode to remove certain soluble impurities. Finally the mixture was filtered through Whatman No. 52 paper to remove the carbon and any other solid COATING THICKNESS TESTING The thickness of the electroplated silver coatings was determined by means of the Aminco-Brenner Magne-Gage.

CATHODE CURRENT EFFICIENCIES The cathode current efficiencies obtained with the various plating procedures were determined, using a copper coulometer in the plating circuit.

BEND TESTING OF STEEL PINS A slow-bend test was used to determine 1. the extent to which the pins had become embrittled during plating, and 2. the quality of the adhesion of the electroplated silver to the basis metal. This test was performed in a Hounsfield notched-bar bending jig attached to a Type-W tensometer machine equipped with a motor drive. The test may be described in the following terms: During the testing, the unnotched pin is supported at two points 3.02 cm. (1.19 in.) apart. Pressure applied to a 0.32 cm. (0.125 in.) diameter mandrel forces the pin into the gap between these points until it breaks or until a bend of is produced at the end of l 1 minutes. The angle at which a pin is fractured, or to which it is bent, is calculated from a chart which operates with the machine. The bending jig originally produced a bend of 90"; however, it was modified to give a maximum bend of 135. Pins that can be bent through this angle in l 1 minutes without breaking are considered to have passed the test for embrittlement.

GENERAL DESCRIPTION OF THE EXAMPLES l=0.374+0.768X-0.l71'3X (Eq. 1) wherein Y=0H normality and X=free CN normality W is between 6.1 and 8.1

Ag normallty (the exact value depending on the bath concentration). A procedure has now been provided for determining the composition of the initial unstable bath (formed by dissolving ""=Slightly unstable.

silver cyanide, an alkali metal cyanide, e.g., sodium cyanide, and an alkali metal hydroxide, e.g., sodium hydroxide, in water) which, after preliminary electrolysis, becomes a stable bath having the desired silver cyanide content. The method as now provided is as follows, remembering that the normalities Cathode current density of the silver and the 0H ion do not change during the conversion of the initial unstable bath into the final stable bath:

a. Decide on the normality of the silver cyanide required in the desired final silver plating bath.

b. Determine the total NaCN normality required in the initial bath, using the silver cyanide normality and the curve in FIG. 2.

c. Determine the free CN normality required in the final stable bath, using the total NaCN normality required in the initial bath and the curve in FIG. 3.

d. Determine the 0H normality required in the original bath, using the free CN normality required in the final bath and the curve in FIG. 1.

Plating was done on sheet cathodes of low-carbon steel in stable baths 4 to 11 (whose analyses are shown below in Table II), to determine the range of cathode current densities that would not produce instability in either bath. The results obtained at 25 C. (77 F.) are presented below in Table III, and 30 7 those obtained at 49 C. (120 F.) below in Table IV. The curve obtained when the total NaCN normality was plotted against the AgCN normality in Baths 4 to 11, is shown in FIG. 2.

TABLE IV [Etlect 0i cathode current density on the stability of typical stable sodium cyanide silver plating baths at 49 C. (120 F.)]

Stable sodium cyanide silver plating baths (amp./dm. 4 5 6 7 8 9 10 v -=Slightly unstable. Q-Stiible.

From the above Tables, it is seen that at 25 C. all of these baths remained stable at current densities up to 3.2 amp./dm. but only Bath 8 remained stable at 17.2 amp./dm. also that increasing the bath temperature to 49 C. (120 F.) increased the maximum current density at which each bath remained stable.

EFFECT OF CATHODE CURRENT DENSITY ON THE CATHODE CURRENT EFFICIENCY OF THE BATHS OF ASPECTS OF THIS INVENTION The plating in Example 11 was done on sheet cathodes of low-carbon steel in stable Baths 7, 9 and 10. The results are tabulated below in Table V.

TABLE II [Analyses of stable sodium cyanide silver plating baths of aspects of this invention (normality)] Constituent 4 5 6 7 B 9 10 16 0. 26 0. 30 0. 0. 40 0. 40 0. 40 .30 l. 84 2. 13 2. 2. 52 2. 65 2. 86 ce 0. 012 0. 030 0. 030 0. 030 0. 040 nd 1. 40 1.70 2. 00 2. 10 2. 20 2. 40 NaOH 1.00 1.06 1.10 1.170 1. 20 1.20 1.20 1. 20

Depends on the purity of the chemicals used. "Not determined.

NOTE.NB2CO3, iree CN-, electrolyzed until it reached the st the limiting value for the particular bath at the temperature used.

[Etiect 0t cathode current density on the stability of typical stable sodium cyanide silver plating baths at 25 C. (77 F.)]

Stable sodium cyanide silver plating baths Cathode current density (ampJdmJ) eeeeee C eeeeeeeeee =stable.

TABLE IV [Eflect of cathode current density on the stability of typical stable sodium cyanide silver plating baths at 49 0. (120 F.)]

Stable sodium cyanide silver plating baths Cathode current density (ampJdm!) and NaOH were determined after the bath had been able condition. The current density used did not exceed TABLE V Effect of Cathode Current Density on Cathode Current Efiiciency of Stable Sodium Cyanide Silver Plating Baths and Unstable Silver Plating Baths (Per Cent) (b) 49IZC (120 C) Cathode Current Efiiciency (per cent) Unstable Silver Plating Bath Cathode Current Density ia pldm l Stable Sodium Cyanide Silver Plating Baths From the above Table, it is seen that the maximum cathode current density at which the cathode current efficiency is 100 percent is greater in the case of Bath 9 than in those of Baths 7 or 10, at both 25 and 49 C. It also is seen that the current efficiency decreases fairly rapidly as soon as the maximum current density is exceeded. it is also interesting to note, from the data in Table V, that the maximum current density at which the current efficiency is 100 percent appears to be much lower in unstable baths than in stable ones.

EXAMPLE Ill Performance of Pins Plated in Baths 4, 5, 6 and 8, in the Slow Bend Test The results of bend tests on pins of Types 1062 and 4037 steels which had been plated with silver in stable baths 4, 5 6 and 8 under suitable conditions, are presented below in Table VI.

12 TABLE vn Initial Composition of Present-Day Unstable Silver Plating Baths (Normality Determined after eonstituentswere dissolved in solution Depends on the purity ot'chemicals used.

TABLE VIII [Comparison of the stability of typical present-day plating baths (1, 2 and 3) with that of typical stable sodium cyanide silver plating baths (1 and 10)] v Cathode Length Plating current of Analysis (normality) temperdensity plating ature (empJ time Free Bath Bath N0. C.) dm!) (hr.) Ag+ CN' OH- stability 0 0. 01 0.80 Trace Trace 1 25 a ---6-.-.--

. race 2 25 as 2-1: 0 0.7 3 49 i 9.0 0.90 0. 3 51.53 011 0 0.16 race 4 2 i 2. 2 0. s 1.08 0 3 T168003 TABLE Vl As seen in Table Vlll, the compositions of baths 1, 2 and 3 were unstable. This was particularly true in the case of Bath 3 Remus of slow Bend Tess on Types 1062 and 4037 which was used at a higher current density for a longer time.

Pins Plated Using Stable Sodium Cyanide Silver Baths 4 and 10 w re stable. Plating Baths Stable Baths EXAMPLE V 4 5 6 s EMBRITTLEMENT OF THE STEEL AND ADHERENCE Plating rt-m 49 49 49 49 OF THE ELECTROPLATED STEEL 12 c". Cathode Current 5.4 11.8 10.0 26.9 Pins of Types 1062 and 4037 high-strength steels were 7 P plated with silver in Baths 1, 2 and 3. Then they were tested 12 Pin 24 l2 n for embrittlement and for adherence of the silver to the steel. The results are tabulated below in Table lX. No.0iPins 3' 24 l2 i2 Unbroken IX Angle of Bend 135 135 l35 135 Before Breaking (degrees) Results of Flow Bend Tests on Types 1062 and 4037 Adhfmncc m Very very very very Pins Plated Using Conventional Unstable Baths Stee good good good good Unstable Bahs Type 1062 pins only.

Plating Temp. 25 25 49 2'= c. From the above Table, it w ll be noted that: Ealhode current 1. none of these plated pins broke when bent through an Density (arnpJdm?) 1.6- 2.2 [0.8 angle of 135 degrees, indicating that they all were free i gl u 12 from significant embrittlement; and gf g z 2 u 4 2. the adherence of the silver to the pins plated in these Angle or Bend baths was very good Before Breaking 58-l32 I35 91-129 (degrees) Adherence to steel Good Very I Very EXAMPLE IV poor poor BATH STABlLITY Unstable Baths 1, 2 and 3 of the prior art and stable sodium cyanide silver plating Baths 4 and 10 were used to electroplate steel, and the composition of each bath was determined before and after plating. The initial composition of Baths Nos. 1, 2 and 3 is given below in Table Vll, while the initial composition of Baths 4 and 10 was previously given in Table ii. The composition of each bath after plating is given below in Table VII].

From this Table, it is seen that the properties of the plated pins from all three baths were very unsatisfactory. in Bath 1 a large proportion of the pins had become severely embrittled; in the case of Bath 2 the silver did not adhere well to the steel; and in Bath 3 both severe embrittlement and poor adherence occurred. These results are to be compared with those shown in Table VI.

EXAMPLE Vl COMPARISON OF OTHER CHARACTERISTICS OF STABLE AND UNSTABLE BATHS Certain other differences between typical stable sodium cyanide silver plating baths and unstable conventional silver cyanide plating baths are tabulated below in Table X.

TABLE X Comparison of some characteristics of stable sodium cyanide silver plating baths and unstable conventional silver plating baths] Conventional potassium Stable sodium cyanide It is seen from Table X that there are the following differences:

1. When unstable baths are used, the plating cell voltage varies during plating, whereas with stable baths it remains constant.

2. When stable baths are used, the anode is always clean and bright, which is not true in the case of unstable baths.

3. When stable baths are used, the anode dissolves only during plating; with unstable baths, the anode also dissolves when plating is not being done.

4. The amount of crystalline material depositing on the anodes and cell walls is very small in the case of the stable baths, regardless of bath concentration; however, the amount depositing is considerably greater in the case of the unstable baths, particularly when they are comparatively concentrated.

GENERAL OBSERVATIONS It has also been observed that when the OH normality of a bath is slightly higher or slightly lower than that required for a stable bath as outlined in Equation l above e.g., 1.2 g./l. higher or lower), the limiting cathode current density is much lower than when the OH" normality agrees exactly with this relationship. When used at or below these lower limiting current densities, the baths remain stable and have characteristics somewhat similar to those of 1the baths which exactly fulfill both of the requirements for stability. However, serious deterioration of the bath occurs if the limiting current density is exceeded.

Baths in which the H normality is considerably higher than that required for stability are unstable at any cathode current density, and pins plated in them become highly em brittled. On the other hand, the adherence of the silver deposits from these baths is fairly good. The same is true of baths in which the 0H normality is considerably lower than that required for stability, except that the adherence of the metal deposits is very poor.

It is further noted that sodium cyanide, which is considerably cheaper than potassium cyanide, was used in the stable baths of aspects of this invention instead of the potassium cyanide. Furthermore, although present day industrial silver plating baths frequently contain one or more addition agents to improve the appearance of the plated metal, such materials were not used in preparing the stable baths of invention.

Thus, by the present invention, there is now provided stable silver cyanide plating baths, certain preferred embodiments of which have the following important advantages:

a. the control of the bath composition is comparatively simple, which usually results in greater uniformity and higher quality in the plated products;

b. the nature and adhesion of the resulting silver coatings are improved;

c. the operation of the plating cell is simpler;

d. higher cathode current densities can be used;

e. high-strength steels, such as Types 1062 and 4037, can be plated with silver without becoming significantly embrittled;

f. silver coatings can be plated directly on steel without the use of a strike bath;

g. anodes do not dissolve in the bath when the latter is not in use; and

h. steel parts, immersed in the plating bath, are not coated with silver by chemical replacement.

It has also been found that the stable silver cyanide plating baths are not stable when first made up. However, they become stable after an initial period of plating during treatment with activated carbon. The method of calculating the composition of a new bath, prior to electrolysis, which, after such electrolysis, provides the stable bath constitutes another aspect of this invention.

It has further been found that such stable silver plating baths can become unstable if too high a cathode current density is used. When this happens the composition of the bath must be adjusted, according to yet another aspect of this invention. During such restabilization treatment, the bath may optionally be treated with activated carbon, depending on the nature of the instability of the bath being restabilized.

The preceding examples can be repeated with similar success by substituting the generically and specifically described reactants and operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

We claim:

I. Stable silver cyanide plating baths comprising ions of silver, ions of sodium and ions of cyanide, the normalities of the constituents of each such bath being related to one another by the following equations:

aspects of this wherein Y=OH normality, and

=free CN normality;

and

AgCN: 0.16 N-0.40 N NaCN (total): 1.30 N2.85 N Free ca. 1.08 N-2.40 N NaOH 1.00 N-l .20 N

3. A process for applying a plating of silver onto a metal surface, which process comprises: electroplating a silver coating onto a cleaned surface of said metal by using a stable silver cyanide plating bath comprising ions of silver, ions of sodium and ions of cyanide, the normalities of the constituents of the bath being related to one another by the following equations:

Y=0.374+0.768X I 78X (I) wherein Y=OH normality, and X=free" CN" normality and Total sodium cyanide normality Total ve aa mal by,

4. The process of claim 3 wherein the metal being coated is copper.

5. The process of claim 3 wherein the metal being coated is steel.

6. The process as claimed in claim 5, further comprising a pretreatment of said steel prior to electroplating said silver coating, said pretreatment producing or maintaining a clean, smooth, substantially oxide-free surface onto steel which is conducive to being electroplated with silver without causing hydrogen embrittlement, and comprising the steps of providing a cleaned surface, smooth surface, substantially free of oxide film, then treating the surface with an acid cleaning solution consisting essentially of nitric acidacetic acidphosphoric acid, thereby to maintain said smooth, clean surface, and finally removing smut from the acid washed steel surface.

7. The process of claim 5, wherein the current density ranges from 1.6 amp./dm. to 26.9 ampJdm.

8. The process of claim 7 wherein the plating temperature is about 18 C. or more.

9. The process of claim 7 wherein the plating cell voltage remains substantially constant during plating.

10. The process as claimed in claim 5, further comprising a pretreatment of said steel prior to electroplating said silver coating, said pretreatment producing or maintaining a clean, smooth, substantially oxide-free surface onto steel which is conducive to being electroplated with silver without causing hydrogen embrittlement, and comprising the steps of cleaning away oxide film from the steel surface, then producing, electrolessly, a thin fugitive porous coating of copper on the steel surface, then removing that metal coating and simultaneously producing a smooth surface by washing with an acid cleaning solution containing nitric acid and acetic acid, and finally removing smut from the steel surface.

11. The process of claim 10 wherein a thin porous fugitive copper coating is formed on said treated steel surface.

12. The process of claim comprising the steps of:

a. cleaning said steel surface with hydrochloric acid;

b. depositing, electrolessly, on said steel a thin porous copper flash;

c. contacting said deposited copper surface with an acid cleaning solution containingnitric acid and acetic acid; and

d. cleaning said surface resulting from step (c) with water.

13. The process of claim wherein the hydrochloric acid cleaning step (a) is conducted in conjunction with ultrasonic vibration.

14. The process of claim 10 wherein the acid cleaning solution comprises nitric acid, about 10 to about percent by weight, acetic acid, about 4 to about 35 percent by weight, balance substantially all water.

15. The process of claim 10 wherein the acid cleaning solution comprises nitric acid, about 23 percent by weight, acetic acid, about 31-35 percent by weight, phosphoric acid, about 27-30 percent by weight, balance substantially all water.

16. The process of claim 10 wherein the step (d) of water washing is accomplished by water rinsing in conjunction with ultrasonic vibration.

17. The process of claim 10 wherein the step (d) of washing with water is conducted by air-water blasting.

= Between 6.1 and 8.1.

18. The process of claim 10 wherein the step (d) of water washing is accomplished with water under pneumatic or hydraulic pressure.

19. The process of claim 10 wherein the step (d) of water washing is accomplished by washing under hydraulic pressure in conjunction with ultrasonic vibration.

20. The process of claim 10 wherein the step (d) of washing with water is conducted by air-water blasting in conjunction with ultrasonic vibration.

21. The process of claim 10 which includes prior to the step of producing the fugitive, porous coating, the following step:

making the steel anodic in 0.05-0.2N HCI for a time less than 10 minutes, the potential of the steel being O.5 to 0.4 volts with respect to a saturated calomel electrode.

22. A process for producing a stable silver cyanide plating bath, the stable plating bath to be produced having a composition satisfying the following conditions, namely, the normalities of the constituents of the bath being related to one another by the following equations:

Y=0.374+0.768X-O. I 78X (I) wherein Y=0H normality, and

X= ree CN normality and Total sodium cyanide normality ...Total s av u n l tya. -w. W the process comprising: dissolving sodium cyanide, silver cya nide and sodium hydroxide in water to form a bath in which, for a predetermined and selected silver cyanide normality, the total sodium cyanide normality is determined from the curve in FIG. 2, in which, for that determined total sodium cyanide normality, the free" CN normality is'determined from the curve in FIG. 3, and in which, from that determined free CN normality, the 0H normality is determined from the curve in FIG. 1; placing silver anodes and metal cathodes in said bath; and slowly agitating and electrolyzing said bath for a period of time until a stable bath is formed in which the free CN normality is constant.

23. The process of claim 22 wherein said metal cathodes are steel.

24. The process of claim 22 wherein said metal cathodes are copper.

25. A process for restabilizing a formerly stable silver cyanide plating bath which comprises: selecting a bath whose composition formerly satisfied the following conditions, namely, that the normalities of the constituents of the bath being related to one another by the following equations:

Between -.1 nd. l;

Total sodium cyanide normality gi ly gim) u Between 6.1 and 8.1

and placing said bath into a container; readjusting the normalities of the constituents so that, for a predetermined and selected silver cyanide normality, the total sodium cyanide normality is determined from the curve in FIG. 2; for that determined total sodium cyanide normality, the free" CN normality is determined from the curve in FIG. 3; and from that determined free" CN normality, the 0H normality, is determined from the curve in FIG. 1; placing a silver anode and a metal cathode into said bath; and slowly agitating and electrolyzing said bath for a period of time until a stable bath is formed in which the free CN normality is constant.

2?. The process of claim 25 wherein said metal cathodes are stee 27. The process of claim 25 wherein said metal cathodes are copper.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 I 6 I 858 Dated February 9 I 1972 Inventor(s) Wilfred Dingley, et a1.

It is certified that/error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE HEADING Under CLAIM OF PRIORITY" insert I CAHANDA 024,569 of JULY 8, I969 IN THE CLAIMS:

CLAIM 1, COLUMN 14 LINE 5 OF THE CLAIM: The end of the equation should read 0.178X2 CLAIM 2, COLUMN 14, LINE 5 OF .THE CLAI he constituent "cB" should read CN I Signed and sealed this 11th day of June 19714..

(SEAL) Attest:

EWARD M.FLETCHER, JR. c MARSHALL mm: Attesting Officer Commissioner of Patents F (10-59) uscoMM-Dc 60376-P6D U45. GOVERNMENT PRINTING OFFICE Z I." O-aifill 

2. The bath of claim 1 comprising the following constituents: AgCN: 0.16 N-0.40 N NaCN (total): 1.30 N- 2.85 N ''''Free'''' CB : 1.08 N- 2.40 N NaOH 1.00 N- 1.20 N
 3. A process for applying a plating of silver onto a metal surface, which process comprises: electroplating a silver coating onto a cleaned surface of said metal by using a stable silver cyanide plating bath comprising ions of silver, ions of sodium and ions of cyanide, the normalities of the constituents of the bath being related to one another by the following equations: Y 0.374+ 0.768X- 0.178X2 (1) wherein Y OH normality, and X ''''free'''' CN normality
 4. The process of claim 3 wherein the metal being coated is copper.
 5. The process of claim 3 wherein the metal being coated is steel.
 6. The process as claimed in claim 5, further comprising a pretreatment of said steel prior to electroplating said silver coating, said pretreatment producing or maintaining a clean, smooth, substantially oxide-free surface onto steel which is conducive to being electroplated with silver without causing hydrogen embrittlement, and comprising the steps of providing a cleaned surface, smooth surface, substantially free of oxide film, then treating the surface with an acid cleaning solution consisting essentially of nitric acid- acetic acid- phosphoric acid, thereby to maintain said smooth, clean surface, and finally removing smut from the acid washed steel surface.
 7. The process of claim 5, wherein the current density ranges from 1.6 amp./dm.2 to 26.9 amp./dm2.
 8. The process of claim 7 wherein the plating temperature is about 18* C. or more.
 9. The process of claim 7 wherein the plating cell voltage remains substantially constant during plating.
 10. The process as claimed in claim 5, further comprising a pretreatment of said steel prior to electroplating said silver coating, said pretreatment producing or maintaining a clean, smooth, substantially oxide-free surface onto steel which is conducive to being electroplated with silver without causing hydrogen embrittlement, and comprising the steps of cleaning away oxide film from the steel surface, then producing, electrolessly, a thin fugitive porous coating of copper on the steel surFace, then removing that metal coating and simultaneously producing a smooth surface by washing with an acid cleaning solution containing nitric acid and acetic acid, and finally removing smut from the steel surface.
 11. The process of claim 10 wherein a thin porous fugitive copper coating is formed on said treated steel surface.
 12. The process of claim 5 comprising the steps of: a. cleaning said steel surface with hydrochloric acid; b. depositing, electrolessly, on said steel a thin porous copper flash; c. contacting said deposited copper surface with an acid cleaning solution containing nitric acid and acetic acid; and d. cleaning said surface resulting from step (c) with water.
 13. The process of claim 10 wherein the hydrochloric acid cleaning step (a) is conducted in conjunction with ultrasonic vibration.
 14. The process of claim 10 wherein the acid cleaning solution comprises nitric acid, about 10 to about 15 percent by weight, acetic acid, about 4 to about 35 percent by weight, balance substantially all water.
 15. The process of claim 10 wherein the acid cleaning solution comprises nitric acid, about 23 percent by weight, acetic acid, about 31- 35 percent by weight, phosphoric acid, about 27- 30 percent by weight, balance substantially all water.
 16. The process of claim 10 wherein the step (d) of water washing is accomplished by water rinsing in conjunction with ultrasonic vibration.
 17. The process of claim 10 wherein the step (d) of washing with water is conducted by air-water blasting.
 18. The process of claim 10 wherein the step (d) of water washing is accomplished with water under pneumatic or hydraulic pressure.
 19. The process of claim 10 wherein the step (d) of water washing is accomplished by washing under hydraulic pressure in conjunction with ultrasonic vibration.
 20. The process of claim 10 wherein the step (d) of washing with water is conducted by air-water blasting in conjunction with ultrasonic vibration.
 21. The process of claim 10 which includes prior to the step of producing the fugitive, porous coating, the following step: making the steel anodic in 0.05- 0.2N HCl for a time less than 10 minutes, the potential of the steel being -0.5 to 0.4 volts with respect to a saturated calomel electrode.
 22. A process for producing a stable silver cyanide plating bath, the stable plating bath to be produced having a composition satisfying the following conditions, namely, the normalities of the constituents of the bath being related to one another by the following equations: Y 0.374+ 0.768X- 0.178X2 (1) wherein Y OH normality, and X ''''free'''' CN normality the process comprising: dissolving sodium cyanide, silver cyanide and sodium hydroxide in water to form a bath in which, for a predetermined and selected silver cyanide normality, the total sodium cyanide normality is determined from the curve in FIG. 2, in which, for that determined total sodium cyanide normality, the ''''free'''' CN normality is determined from the curve in FIG. 3, and in which, from that determined ''''free'''' CN normality, the OH normality is determined from the curve in FIG. 1; placing silver anodes and metal cathodes in said bath; and slowly agitating and electrolyzing said bath for a period of time until a stable bath is formed in which the ''''free'''' CN normality is constant.
 23. The process of claim 22 wherein said metal cathodes are steel.
 24. The process of claim 22 wherein said metal cathodes are copper.
 25. A process for restabilizing a formerly stable silver cyanide plating bath which comprises: selecting a bath whose composition formerly satisfied the following conditions, namely, that the normalities of the cOnstituents of the bath being related to one another by the following equations: Y 0.374+ 0.768X- 0.178X2 (1) wherein Y OH normality, and X ''''free'''' CN normality and placing said bath into a container; readjusting the normalities of the constituents so that, for a predetermined and selected silver cyanide normality, the total sodium cyanide normality is determined from the curve in FIG. 2; for that determined total sodium cyanide normality, the ''''free'''' CN normality is determined from the curve in FIG. 3; and from that determined ''''free'''' CN normality, the OH normality, is determined from the curve in FIG. 1; placing a silver anode and a metal cathode into said bath; and slowly agitating and electrolyzing said bath for a period of time until a stable bath is formed in which the ''''free'''' CN normality is constant.
 26. The process of claim 25 wherein said metal cathodes are steel.
 27. The process of claim 25 wherein said metal cathodes are copper. 